1. Field of the Invention
The invention relates to a method and circuit for deciding a data transfer rate and, more particularly to, a method and circuit for deciding a data transfer rate of the Ethernet (registered trade name) via a serial bus according to the IEEE 1394 Standard.
2. Description of the Related Art
According to the IEEE 1394 Standard, two pairs of twisted-pair signals (TPA/TPA*, TPB/TPB*) are used to carry out all the processes such as transmission/reception of data and arbitration between devices asking for assignment of the same resources. To interconnect apparatuses, a cross cable is used. That is, one of the TPA/TPA* terminals is connected with the other of the TPB/TPB* and, conversely, one of the TPB/TPB* terminals is connected with the other of the TPA/TPA* terminals.
Also, the IEEE 1394 Standard provides a plurality of data transfer rates of 100 Mbps, 200 Mbps, 400 Mbps, etc. The data transfer rate is notified of at the time of arbitration performed before transmission/reception of a data packet, when both the transmission side apparatus and the reception side apparatus pull a common mode signaling current through the TPB/TPB* terminals to then change a common mode voltage. Since they have been connected with a cross cable, conventionally both of the transmission side and reception side apparatuses have compared the common mode voltages at the respective TPA/TPA* terminals to each other, thus deciding the data transfer rate.
By this data transfer rate notification method, however, a common mode voltage is changed to decide a data transfer rate based on thus changed common mode voltage. In this case, however, the transmission side and reception side apparatuses may have different characteristics due to fluctuations in manufacture thereof and, therefore, may decide different data transfer rates. Also, the above-mentioned configuration is equipped with only a mechanism for notifying of a data transfer rate but not with means for deciding coincidence of the data transfer rate between the transmission side and the reception side, which has been a problem.
Also, to change a common mode voltage for notification of a data transfer rate, a common mode signaling current must be pulled in, thus contributing to an increase in a current dissipation.
Further, the above problem has occurred also on the Ethernet (registered trade name), which is another related art.